Subterranean drilling operations are procedures conducted below the surface of the earth from a formation such as in the course of recovering oil, gas, and other substances. Typically, rotary subterranean drilling operations involve attaching a drill bit on a lower end of a drill string to form a drilling tool and rotating the drill bit along with the drill string into a subterranean formation to create a well bore through which subsurface formation fluids may be recovered. During subterranean drilling operations, subterranean well bores may encounter with shales, which make up over 75% drilling formation. Shale instability results in significant borehole problems including hole collapse, tight hole, stuck pipe, poor hole cleaning, etc. Reactive shales are materials like clays (e.g. bentonite) and other subterranean materials that swell when exposed to water. Reactive shales can cause shale instability problems during drilling because of their tendency to degrade and react with water when exposed to aqueous-based drilling fluids. This degradation can result in undesirable drilling conditions because degradation of the shale may interfere with attempts to maintain the integrity of drilled cuttings traveling up the well bore until the cuttings can be removed by solids control equipment located at the surface. Degradation of drilled cuttings prior to their removal at the surface greatly prolongs drilling time because the shale particles traveling up the well bore break up into smaller particles. These smaller particles increasingly expose new surface area of the shale to the drilling fluid, which leads to still further absorption of water and further degradation.
Consequently, subterranean drilling operations generally employ drilling fluids to inhibit shale swelling and improve environmental performance. Drilling fluids used in such operations may be fluids (gaseous or liquid), mixtures of fluids, and solids (solid suspensions, emulsions, gases and solids). Drilling fluids are also used to cool the drill bit, lubricate the rotating drill, prevent blowouts, and remove drill cuttings from the well bore.
Numerous patents and published patent applications describe the techniques or products that can be used to inhibit the swelling of clays and shales. The swelling inhibiting compositions disclosed include inorganic phosphates (U.S. Pat. No. 4,605,068); polyalkoxy diamines and their salts (U.S. Pat. Nos. 6,484,821; 6,609,578; and 6,247,543 and U.S. Pub. No. 2003/0106718); choline derivatives (U.S. Pat. No. 5,908,814); oligomethylene diamines and their salts (U.S. Pat. No. 5,771,971 and U.S. Pub. No. 2002/0155956); the addition product of carboxymethyl cellulose and an organic amine (WO 2006/013595); 1,2-cyclohexanediamine and/or their salts (WO 2006/013597); salts of phosphoric acid esters of oxyalkylated polyols, (WO 2006/013596); the combination of a partially hydrolyzed acrylic copolymer, potassium chloride and polyanionic cellulose (U.S. Pat. No. 4,664,818); quaternary ammonium compounds (U.S. Pat. No. 5,197,544); polymers based on dialkyl aminoalkyl methacrylate, (U.S. Pat. No. 7,091,159); aqueous solutions containing a polymer with hydrophilic and hydrophobic groups, (U.S. Pat. No. 5,728,653); the reaction product of a polyhydroxyalkane and an alkylene oxide (U.S. Pat. No. 6,544,933). The shale swelling inhibitors are also disclosed in U.S. Pat. Nos. 8,258,085; 7,087,554; 7,549,474; 7,833,945; and 6,620,769. Further background information can be found in A. V. Reis, et al, J. Org. Chem., 2009, 74, 3750-3757; U.S. Pat. No. 5,037,930; S. H. Kim, et al, Carbohydrate Polymers 40 (1993) 180-190; and Fhilype J., et al, International Journal of Pharmaceutics, 355 (2008) 184-194.
Conventional methods for inhibiting shale swelling during drilling have widely adopted oil and synthetic based drilling fluids. However, water based shale swelling drilling inhibitors are now increasingly employed as they are more environmentally friendly. Many of such shale swell inhibitors show good inhibition performance in the oil based drilling operations but their applications are limited due to their insufficient biodegradability. Therefore, there is a need for shale swelling inhibitors having enhanced biodegradability.
U.S. Pat. No. 8,258,085 discloses that an aqueous fluidized polymer suspension for use in oilfield applications comprises an allyloxy based co-polymer; a water soluble polymer; a salt; and water, wherein the water soluble polymer is selected from the group consisting of hydroxyethyl cellulose (HEC), carboxymethyl cellulose (CMC), guar, guar derivatives and xanthan and wherein the allyloxy based co-polymer and the water soluble polymer are dispersed in the water containing the salt.
U.S. Pat. No. 6,620,769 discloses an oil-based fluid polymer suspension (FPS) composition for use in oil or gas well servicing fluids comprising (a) a hydrophilic polymer, (b) an organophilic clay, (c) a stabilizer that is a non-ionic surfactant and (d) a white medicinal oil that is non-toxic and biodegradable 60% at least after 28 days in freshwater and seawater under OECD 301 and OECD 306 test protocols, wherein the FPS composition is environmentally acceptable for use in onshore and offshore oil field servicing fluids.
GB Patent No. 2,267,921 discloses a drilling fluid system containing a homopolymer of polyvinylpyrrolidone (PVP) used as a shale inhibiting additive, wherein the PVP is present at concentrations of 0.01% to 5.0% by weight.
U.S. Pat. No. 4,142,595 discloses a clay-free aqueous drilling fluid comprising: (a) 0.2 to 1.5 pounds per barrel of flaxseed gum; (b) a predetermined amount of at least one salt having a cation selected from the group consisting of potassium and ammonium, said amount providing a cation concentration in said drilling fluid of at least 10,000 ppm; and (c) a non-clay viscosifier selected from the group consisting of carboxymethyl cellulose, hydroxyethyl cellulose (HEC), and hydroxyethyl cellulose plus a heteropolysaccharide produced by the bacterium Xanthomonus campestris NRRL B-1459.
In view of the foregoing, it is an object of the present application to provide environmentally friendly and biodegradable shale swell inhibitors which have high shale recovering ability during oil and gas based drilling operations.
Serendipitously, it has been discovered that hydroxyethyl cellulose (HEC), a non-ionic water soluble polymer blended with crosslinked polyvinylpyrrolidone can demonstrate excellent shale swell inhibition properties.